A shell and tube heat exchanger includes two tube sheets and a plurality of tubes connecting the tube sheets and is configured to be enclosed by a cylindrical body, i.e., a shell. The shell and tube heat exchanger is used in various types of heat exchange, such as heating, cooling, condensation, evaporation, etc.
In general, a shell and tube heat exchanger is configured such that different fluids are introduced into and discharged from a tube side and a shell side and the fluid at the tube side and the fluid at the shell side exchange heat, as exemplarily shown in FIG. 1. As a movement path of the fluid at the shell side, the fluid at the shell side is introduced into a shell side inlet (S-S-I), zigzags through a plurality of baffle plates 30 formed in the shell, and is discharged to the outside through a shell side outlet (S-S-O). The tubes 10 are welded to tube sheets 20 provided at both sides of the shell so as to prevent mixing of the fluid in the tube 10 and the fluid in the shell. As a movement path of the fluid at the tube side, the fluid at the tube side is introduced into a tube side inlet (T-S-I), introduced into the tubes 10 passing through the inside of the shell, exchanges heat with the fluid in the shell, and is then discharged to the outside through a tube side outlet (T-S-O).
Here, in order to prevent mixing of the different fluids at the shell side and at the tube side, the tubes 10 passing through the inside of the shell should be completely isolated from the inside of the shell and, thus, the tubes 10 are welded to two tube sheets 20 symmetrically formed at both sides of the shell to face each other.
As a prior art document (patent document 1), Korean Patent Registration No. 10-1298703 discloses a welding method of the tube sheets 20 and the tubes 20 of the shell and tube heat exchanger. That is, as exemplarily shown in FIG. 1, a plurality of tube insertion holes H is formed on the tube sheets 20, and the tubes 10 are expanded under the condition that the tubes 10 are respectively inserted into the tube insertion holes H of the tube sheets 20, and thus fixed to the tube sheets 20. Thereafter, tube side welding is carried out between the tubes 10 and the tube sheets 20, thereby combining the tubes 10 and the tube sheets 20. Here, tube side welding refers to welding to prevent a fluid introduced into and discharged from the tubes 10 from permeating into gaps between the tubes 10 and the tube sheets 20, and is performed at outer regions (i.e., regions contacting the fluid introduced into and discharged from the tube side) when the tubes 10 and the tube sheets 20 are combined.
Although the tubes 10 are expanded and then tube side welding is carried out, the tube sheets 20 and the tubes 10 at the shell side do not completely contact each other, fine gaps therebetween are generated, and cracks are formed in the tube 10 due to corrosion through the gaps. In order to overcome such a problem, tube grooves 12 and tube sheet grooves 22 are respectively formed on the tube sheets 20. The tube groove 12 receives a designated portion of the tube 10 when the tube 10 is expanded and thus serves to increase combining force, and the tube sheet groove 22 serves to facilitate shell side welding.
The tube groove 12 is formed on the inner surface of the tube insertion hole H of the tube sheet 20 and allows the tube 10 to be more closely combined with the tube sheet 20 when the tube 10 is inserted into the tube insertion hole H and then expanded. Thereafter, shell side welding is carried out. Shell side welding refers to welding to prevent a fluid introduced into the shell side from permeating into gaps between the tubes 10 and the tube sheets 20 so as to prevent generation of cracks in the tube 10 and corrosion of the tube 10 due to the fluid, and is performed at inner regions (i.e., regions contacting the fluid introduced into and discharged from the shell side) when the tubes 10 and the tube sheets 20 are combined. The tube sheet grooves 22 are formed on the inner surfaces of the tube sheets 20 and shell side welding is carried out using a welding torch T. The tube 10, i.e., a base material, is directly welded to a tube sheet lower end joint part 23 at the outside of the tube sheet 20 by the welding torch T.
Here, the reason why the tube sheet grooves 22 are formed on the side surface of the tube sheet 20 is that, during shell side welding, it is not necessary to heat the entirety of the tube sheet 20 but only the tube sheet lower end joint parts 23 formed at the tube sheet grooves 22 may be heated to achieve welding. Since the thickness of the tubes 10 is about 1.5 to 2 mm, if the tube sheet lower end joint parts 23 formed by the tube sheet grooves 22 have the same thickness as the tubes 10, although welding may be carried out by locally heating only the tube sheet lower end joint parts 23 not by heating the entirety of the tube sheet 20, thereby reducing consumption of heat required for welding and thus increasing welding efficiency. Further, the tube sheet groove 22 formed on the side surface of the tube sheet 20 is spaced from the tube insertion hole H, into which the tube 10 is inserted, by a predetermined distance and formed in an arc shape around the tube insertion hole H.
Welding between the tube sheets 20 and the inner sides of the tubes 10 and welding between the tube sheets 20 and outer sides of the tubes 10, i.e., tube side (outer side) welding and shell side (inner side) welding between the tube sheets 20 and the tubes 10, are respectively carried out and, here, any one of tube side (outer side) welding and shell side (inner side) welding may be carried out first.
However, in the prior art document (patent document 1), during shell side welding, the tubes 10 are directly welded to the tube sheet lower end joint parts 23, formed by the tube sheet grooves 22, at the outside of the tube sheet 20 by the welding torch T and, in this case, several tens to hundreds of tubes 10 are welded to the shell side of the tube sheet 20. Therefore, direct welding of the tubes 10 to the outer side of the tube sheet 20 is desirable but, in order to weld a plurality of tubes 10 to the tube sheet 20, it is difficult to assure a space for access of the welding torch T to the tube sheets 20 and thereby regions in which welding is not possible are generated. That is, in order to directly weld the outer sides of the tubes 10 to the shell side of the tube sheet 20 using the welding torch T, welding of tubes 10 located at the edge region of the tube sheet 20 is possible but, when other adjacent tubes 10 or other tubes 10 located at the central region of the tube sheet 20 are welded to the tube sheet 20, interference between the tube 10 and the tube 10 occurs and thus welding is difficult, welding efficiency is lowered and complete welding operation is difficult.
As another prior art document (patent document 2), Korean Patent Laid-open Publication No. 10-2013-0081440 is described. That is, as exemplarily shown in FIG. 2, in order to prevent generation cracks and corrosion in tubes 10 due to permeation of a fluid at a shell side into gaps between tubes 10 and tube sheets 20, welding W is carried out by inserting a welding torch T into a tube 10 during shell side welding. The welding torch T is inserted into the tube 10, welding heat is first applied to the tube 10, i.e., a base material, and indirectly transmitted to a tube sheet lower end joint part 23 formed by a sheet groove 22 via the tube 10, thereby performing welding between the tube 10 and the tube sheet lower end joint part 23 of the tube sheet 20.
Here, the reason why the welding torch T is inserted into the tube 10 to perform welding is that, if a plurality of tubes 10 inserted into the tube sheet 20 is welded directly using the welding torch T at the outside of the shell side, it is difficult for the welding torch T to access a narrow space between the tube 10 and the tube 10 and, even if welding is carried out using the welding torch T, a defect may be generated and thus the welding torch T is inserted into the tube 10 to perform welding.
Further, welding between the tube sheets 20 and the inner sides of the tubes 10 and welding between the tube sheets 20 and outer sides of the tubes 10, i.e., tube side (outer side) welding and shell side (inner side) welding between the tube sheets 20 and the tubes 10, are respectively carried out and, here, any one of tube side (outer side) welding and shell side (inner side) welding may be carried out first.
In the prior art document shown in FIG. 2 (patent document 2), during shell side welding, the welding touch T is inserted into the tube 10 to perform welding under the condition that the tubes 10 are inserted into the tube sheets 20. However, since welding heat of the welding touch T is not directly transmitted to the tube sheet lower end joint part 23 of the tube sheet 20 welded to the outer circumferential surface of the tube 10 but welding heat is indirectly transmitted to the tube sheet lower end joint part of the tube sheet 20 via the tube 10, cracks in the tube 10 and welding defects may be generated due to a difficulty in accurate transmission of welding heat to the tube sheet lower end joint part 23, thus lowering productivity.
Further, in the above-described conventional prior arts, since the tube grooves 12 are formed on the tube sheets 20 and, when the tubes 10 are expanded, designated portions of the tubes 10 are inserted into the tube grooves 12 to increase combining force, the tubes grooves 12 should be separately formed on the tube sheets 20, a process of expanding the tubes 10 is required and, thus, manufacturing costs are increased.
In order to solve such problems, as yet another prior art document (patent document 3) filed by the applicant, Korean Patent Registration No. 10-1359778 entitled “Welding method of shell and tubes” is given. However, in addition to the above-described prior art documents, patent document 3 still has problems, such as no use of a welding rod caused by possibility of only welding between base materials due to a narrow spatial limitation, impossibility of acquiring the best weld quality, oxidization or nitrification and corrosion due to extended use.